In the production environment, RDMA gets popular and common for a
networking card. So support RDMA as transport layer protocol to
improve performance and cost reduction.

Note that this feature is ONLY implemented/tested on Linux.

Actually, this is the v2 implementation. The v1 uses low level IB
verbs API directly, the code and discuss ses PR:
    redis#9161

Instead of low level API, the v2 use rsocket which is implemented
by rdma-core to simplify the work in Redis.

The test result is quite exciting:
CPU: Intel(R) Xeon(R) Platinum 8260.
NIC: Mellanox ConnectX-5.
Config of redis: appendonly no, port 6379, rdma-port 6379, appendonly no,
                 server_cpulist 12, bgsave_cpulist 16.
For RDMA: ./redis-benchmark -h HOST -c 30 -n 10000000 -r 1000000000 \
          --threads 8 -d 512 -t ping,set,get,lrange_100 --rdma
For TCP: ./redis-benchmark -h HOST -c 30 -n 10000000 -r 1000000000 \
          --threads 8 -d 512 -t ping,set,get,lrange_100

====== PING_INLINE ======
    TCP: QPS: 159017   AVG LAT: 0.183
v1 RDMA: QPS: 523944   AVG LAT: 0.054
v2 RDMA: QPS: 492683   AVG LAT: 0.052

====== PING_MBULK ======
    TCP: QPS: 162256   AVG LAT: 0.179
v1 RDMA: QPS: 509839   AVG LAT: 0.056
v2 RDMA: QPS: 532226   AVG LAT: 0.048

====== SET ======
    TCP: QPS: 154700   AVG LAT: 0.187
v1 RDMA: QPS: 492368   AVG LAT: 0.058
v2 RDMA: QPS: 295534   AVG LAT: 0.095

====== GET ======
    TCP: QPS: 159022   AVG LAT: 0.182
v1 RDMA: QPS: 525099   AVG LAT: 0.054
v1 RDMA: QPS: 411488   AVG LAT: 0.065

====== LPUSH (needed to benchmark LRANGE) ======
    TCP: QPS: 142537   AVG LAT: 0.207
v1 RDMA: QPS: 395038   AVG LAT: 0.073
v2 RDMA: QPS: 353232   AVG LAT: 0.079

====== LRANGE_100 (first 100 elements) ======
    TCP: QPS:  36171   AVG LAT: 0.657
v1 RDMA: QPS:  55266   AVG LAT: 0.412
v2 RDMA: QPS:  52228   AVG LAT: 0.468

Signed-off-by: zhenwei pi <pizhenwei@bytedance.com>